ABSTRACT
The heteromorphic life cycle of the freshwater hypotrich Oxytricha bifaria comprehends at least 3 phases: conjugating pairs, resting cysts and carnivorous giants. The gigantic forms represent unique adaptive devices which enable the species to survive in a certain environment when the normal bacterial food is exhausted. In this report several results are described which deal with the nature of the mechanisms underlying giant formation: (a) only cell to cell contacts (not species-specific) occurring among living specimens trigger this differentiation; (b) the higher the number of cells per ml, the shorter the "induction period", namely the time lag between the onset of the inducing conditions and the formation of the first giant; (c) it is now possible to separate and to distinguish experimentally, within the "induction period", two successive steps, namely "activation" and "predation"; (d) also the already-differentiated-giants (the so-called steadystate-giants) need cell to cell contacts to maintain their cell differentiation. The "Labile Memory Counter" hypothesis is proposed and discussed.
ABSTRACT
The induction period of Oxytricha bifaria, namely the time lag elapsing between the beginning of the inducing (= overcrowded) conditions and the formation of the first giant was studied. It was found that this period consists of at least three successive steps: (a) the "early activation window", roughly corresponding to the first third of the induction period, during which the cell to cell contacts triggering the differentiation must occur; (b) the "late activation window", which corresponds to the central part of the induction period and during which a specific protein synthesis occurs. These two steps end in the "activation" of Oxytricha, which is now ready for the third phase, (c) "prédation" which occurs during the last third of the induction period, when the activated, potential giants actually engulf their preys.
